The identification of the neu and ras protooncogenes, the elucidation of the role of p53 tumor suppressor genes, and the sequencing of BRCA1 and BRCA2 breast cancer susceptibility genes, marked a significant breakthrough for the understanding of the genetic basis of breast cancer. Although the causes of these genetic mutations are not well understood, several factors are currently accepted as increasing the risk of breast cancer, including genetic predisposition and environmental exposure. Several chemicals commonly found in ground water and commercial procedures have been implicated in initiating a variety of cancers, including breast cancer. These chemicals include arsenic, cadmium, copper, mercury, nickel, and organophosphate pesticides. Unlike the carcinogenic nitroso compounds known to initiate and/or promote tumor formation in rodents, heavy metals and ions have been shown to cause tumors in rodents with single injections. Similarly, breast cancer, or susceptibility to it, reportedly requires decades for development. Consequently, the initiation of a sequence of mutational events, triggered by chronic, low-dose, non-carcinogenic yet toxic chemicals, may be the basis for mammary cell transformation, as recent evidence in Suffolk County (NY State) suggests. This proposal, therefore, outlines a series of experiments directed at studying mammary epithelial cells and established breast cancer cell lines using cytotoxicity testing methods and molecular biology techniques. The results will improve our understanding of the interaction between mammary cells and heavy metals and the mechanism by which these chemicals initiate mutations and allow for tumor promotion.